Characterisation of aged HDPE pipes from drinking water distribution : investigation of crack depth by Nol ring tests under creep loading

International audienceHDPE pipes are used for the transport of drinking water. However, disinfectants in waterseem to have a strong impact on their mechanical behaviour, limiting their lifetime inoperation. Indeed, oxidation occurs when they are in contact with disinfectants leading to theformation of a thin oxidised layer coupled to the cracks initiation of cracks of different lengthsfrom the inner wall surface. An original method is proposed here to characterise the ageingeffect of the pipe mechanical behaviour. Inspired from the ASTM D 2290-04 standard, NolRing tests have been performed under tensile and creep loadings on smooth rings. Aconstitutive equation has been determined from these tests using a finite element (FE)modelling. FE simulations have been performed to study the influence of the thin oxidised PElayer. Precracked specimens with different crack depth ratio have also been modelled. Thecrack depth ratio is an important parameter to quantify pipe ageing

Fracture mechanics of creeping solids applied to pre-cracked NOL ring specimens to predict residual lifetime of high density polyethylene pipes

International audienceHigh Density Polyethylene (HDPE) is widely used for the distribution of drinking water and are exposed to an internal pressure due to water flow. Furthermore, when they are in contact with disinfectants, oxidation of HDPE occurs at the immediate surface of the inner wall. This leads to a decrease in the HDPE molar mass and consequently, to a hardening as well as an embrittlement of the material. The oxidised layer thickness seems to stabilize at 200 μm whatever the initial pipe thickness due to the diffusion of reactive species in the inner wall. Inspections with a scanning electron microscope (SEM) of the inner wall of pipes collected on site, after several years of service, showed a network of cracks. The most noxious (deepest) longitudinal crack propagates under the steady internal pressure until the complete failure of the pipe. The experimental investigations consisted of creep crack growth tests, carried out on an original geometry for this type of test using NOL ring. These specimens were cut from the pipes and an internal longitudinal crack implanted. Creep crack growth tests were performed at various net stresses and at various crack depth ratios. At the end of each test, the time to failure was recorded. Before applying the theory of fracture mechanics of creeping solids, 3D finite element simulations were carried out to assess the suitability of assuming plane strain conditions. To this end, a porous viscoplastic model was implemented into an in-house finite element code. A fracture criterion based on critical porosity allowed for the simulation of creep crack growth. The localization of the maximum damage at mid-thickness and the thickness reduction were well captured. In these conditions, the crack front curvature is located near the surface so that a 2D calculation with plane strain conditions can then be justified to determine the C* load parameter. The residual lifetime of a real pipe containing a longitudinal defect, under in-service loading was estimated by using the correlation between the time to failure and the C*-integral

Kinetics of chlorine-induced polyethylene degradation in water pipes

The presence of chlorine in drinking water supplies in many countries creates the undesirable side effect of causing a relatively under investigated degree of polymer degradation in the polyethylene pipes used for transport. In order to predict pipe lifetimes and ensure safe water supplies, a kinetic model using data for the degradation rates of polyethylenes immersed in chlorine solutions, was developed. In order to replicate phenomena that normally occur very slowly at low concentrations of chlorine, accelerated ageing studies were necessary. These were carried out at high chlorine concentrations under well-defined experimental conditions (70, 400 and 4000 ppm). Results showed that, for the chlorine concentrations studied, a chain scission process associated with carbonyl formation is occurring. It was also shown that the rate of this degradation does not depend on the presence of stabilizer. A kinetic model, taking into account the chlorine concentration, is proposed in order to simulate the molar mass changes occurring. This will facilitate the prediction of the degree of polyethylene embrittlement and ultimately the lifetime

Dégradation chimique du PE et influence sur le comportement, l'endommagement et la rupture en fluage : application à la durabilité des canalisations sous pression

Co-encadrement de la thèse : Bruno FayolleHigh density polyethylene pipes (HDPE) appear to be more and more significant in drinking water distribution networks. A good microbiological quality of the distributed water is ensured by adding disinfectants, such as chlorine. The durability of these pipes, initially designed for a 50 years lifetime, is a great challenge for Veolia, which requires a realistic renewal plan. For this purpose, two approaches have been developed: the first one predicts the physico-chemical state of PE after changes due to the contact with chlorine, while the second one estimates the residual lifetime of a pipe, containing cracks initiated by aging. The kinetic scheme of chemical degradation results from the thermooxidation of PE, to which radical initiating reactions due to chlorine were added. Kinetic rates and diffusion coefficients are determined from the experimental results obtained on accelerated aged samples. The influence of oxidation on PE mechanical response under creep loading is then studied from two grades of PE, representatives of two degradation states: the initial material and an aged one. Damage and failure mechanisms are determined according to the degree of aging. The mechanical approach aims at assessing the creep crack growth of a defect initiated after the oxidation process at the inner surface in contact with disinfectants. Two methodologies are suggested to estimate the residual lifetime of a pipe: the global approach, which relies on a master curve C* = f(tR); and the local approach which takes the damage mechanisms into account to model the failure by using finite element analysis. This latter requires constitutive equations, which are based on the experimental results obtained at both macroscopic and microscopic scales.Les canalisations en polyéthylène haute densité (PEHD) prennent une part de plus en plus importante dans les réseaux de distribution d'eau potable. Pour assurer une bonne qualité microbiologique de l'eau distribuée, des agents désinfectants, comme le chlore, sont introduits dans les réseaux. La durabilité de ces tuyaux, initialement prévus pour durer 50 ans, est un enjeu capital pour Veolia, qui souhaiterait disposer d'un modèle de prédiction de durée de vie, tenant compte à la fois, des mécanismes de dégradation du PE au contact du chlore et de l'impact sur la tenue mécanique du tube. Pour cela, deux modèles physiquement motivés sont proposés : l'un prédit l'état physico-chimique du PE suite à sa dégradation au contact d'agents chlorés, tandis que l'autre estime la durée de vie résiduelle d'un tube dégradé, initialement fissuré. Le modèle cinétique de dégradation chimique est issu de la thermo-oxydation du PE auquel des réactions d'amorçage radicalaire dues au chlore ont été ajoutées. Les constantes de vitesse et les coefficients de diffusion sont déterminés à partir des résultats expérimentaux sur échantillons vieillis de façon accélérée. L'impact de l'oxydation sur le comportement mécanique en fluage du PE est étudié à partir de matériaux modèles représentatifs d'un état neuf et d'un état vieilli. Les mécanismes d'endommagement et de rupture sont analysés en fonction du degré de vieillissement. Le modèle mécanique s'intéresse à la propagation d'une fissure, amorcée par l'oxydation sur la surface en contact avec le désinfectant. Deux méthodologies sont proposées pour prédire la durée de vie résiduelle d'un tube : l'approche globale qui repose sur une courbe maîtresse C* = f (tR), et l'approche locale qui s'inspire des mécanismes d'endommagement et modélise ainsi la rupture grâce à un code de calcul par éléments finis. La loi de comportement utilisée dans ce cas s'appuie sur les résultats expérimentaux obtenus aux échelles macroscopique et microscopique

Dégradation chimique du PE et influence sur le comportement, l'endommagement et la rupture en fluage (application à la durabilité des canalisations sous pression)

Les canalisations en polyéthylène haute densité (PEHD) prennent une part de plus en plus importante dans les réseaux de distribution d'eau potable. Pour assurer une bonne qualité microbiologique de l'eau distribuée, des agents désinfectants, comme le chlore, sont introduits dans les réseaux. La durabilité de ces tuyaux, initialement prévus pour durer 50 ans, est un enjeu capital pour Veolia, qui souhaiterait disposer d'un modèle de prédiction de durée de vie, tenant compte à la fois, des mécanismes de dégradation du PE au contact du chlore et de l'impact sur la tenue mécanique du tube. Pour cela, deux modèles physiquement motivés sont proposés : l'un prédit l'état physico-chimique du PE suite à sa dégradation au contact d'agents chlorés, tandis que l'autre estime la durée de vie résiduelle d'un tube dégradé, initialement fissuré. Le modèle cinétique de dégradation chimique est issu de la thermo-oxydation du PE auquel des réactions d'amorçage radicalaire dues au chlore ont été ajoutées. Les constantes de vitesse et les coefficients de diffusion sont déterminés à partir des résultats expérimentaux sur échantillons vieillis de façon accélérée. L'impact de l'oxydation sur le comportement mécanique en fluage du PE est étudié à partir de matériaux modèles représentatifs d'un état neuf et d'un état vieilli. Les mécanismes d'endommagement et de rupture sont analysés en fonction du degré de vieillissement. Le modèle mécanique s'intéresse à la propagation d'une fissure, amorcée par l'oxydation sur la surface en contact avec le désinfectant. Deux méthodologies sont proposées pour prédire la durée de vie résiduelle d'un tube : l'approche globale qui repose sur une courbe maîtresse C* = f (tR), et l'approche locale qui s'inspire des mécanismes d'endommagement et modélise ainsi la rupture grâce à un code de calcul par éléments finis. La loi de comportement utilisée dans ce cas s'appuie sur les résultats expérimentaux obtenus aux échelles macroscopique et microscopique.PARIS-MINES ParisTech (751062310) / SudocSudocFranceF

Effects of aging on the creep behaviour and residual lifetime assessment of polyvinyl chloride (PVC) pipes

International audienceCreep crack growth has been investigated in a polyvinyl chloride (PVC) on circumferentially cracked round bars, on both virgin and aged pipes after 22 and 35 years service. The aim is to predict the resistance to creep failure of pipes under internal pressure by using the fracture mechanics for creeping solids (FMCS) tools. The approach consists of carrying out creep tests on both smooth and cracked specimens. This experimental database is utilised in order to demonstrate the effect of aging on the creep response of the material. A master curve is then plotted, allowing the creep lifetime assessment of laboratory specimens. On the basis of the master curve and under FMCS concept, a methodology for predicting creep failure of cracked pipes is suggested

Proof testing and lifetime reliability of carbon fibre reinforced composite pressure vessels

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Ring tests on high density polyethylene : full investigation assisted by finite element modeling

International audienceIn order to characterize the mechanical behavior of HDPE pipes, the ASTM D 2290-04 standard recommends carrying out tensile tests on notched rings, cut out from the pipe. This very simple test is also utilized to investigate the aging effect of the pipe by determining the strain at failure. Comparison between full ring and notched ring mechanical responses are discussed. Constitutive modeling including strain rate effects was performed by finite element analysis. This allowed a better understanding of the stress state in the cross section perpendicular to the loading direction. Additionally, the influence of a thin layer of oxidized HDPE in the inner wall of the ring was studied in the light of the finite element results

Experimental study of the crack depth ratio threshold to analyze the slow crack growth by creep of high density polyethylene pipes

International audienceTo assess the durability of drinking water connection pipes subjected to oxidation and slow crack growth, a comprehensive database was constructed on a novel specimen geometry: the pre-cracked NOL ring.135 tests were carried out consisting of initial crack depth ratio ranging from 0.08 to 0.6; single or double longitudinal cracks: tensile with steady strain rate and creep loading. A threshold value of the crack depth ratio of 0.2, induced by the oxidation was determined by analyzing several mechanical parameters. This threshold value was shown to be independent on the strain rate effects, single or double crack configuration and the kind of loading: tensile or creep. Creep test results with crack depth ratio larger than 0.2 were then utilized to establish a failure assessment diagram. A methodology allowing the prediction of residual lifetime of in-service pipes was proposed, using this diagram

Contrôle santé par fibres optiques de réservoirs composites pour le stockage d’hydrogène sous haute pression – Projet Horizon Hydrogène Énergie (Projet H2E)

International audienceThis document presents the results of mechanical tests performed on carbon fibre epoxy composite samples of high pressure storage hydrogen vessels, equipped with embedded Optical Fibre Sensors devoted to their Structural Health Monitoring (SHM).Early measurements on flat samples have proved the Fibre Bragg Gratings (FBG) transducers ability to measure strains into such anisotropic materials, and pointed out the optical fibre location importance relative to the structural layers orientations for strain measurements of the structure.Comparative tests on flat samples and NOL rings, with and without embedded optical fibres, did not permit to conclude about any significant intrusive effect of optical fibres on their ultimate tensile strength.Moreover, interpretation in energy of fatigue tests performed on NOL rings, known to be more representative of pressure vessels, have first demonstrated the weakness of the sole interpretation of force measurements which can lead to wrong interpretations, and second the need to accurately control the energy during the first fatigue cycles, since it has a significant effect on the sample fatigue strength.Mechanical tests on vessels first performed to acquire some knowledge about their behaviour, and also to early detect the occurrence of “critical” structural defects, have demonstrated the essential contribution in this application of the Optical Frequency Domain Reflectometry (OFDR) based on Rayleigh backscattering compared to other “traditional” optical techniques, e.g. Brillouin reflectometry.Based on the fact that pressure vessels are subjected to an homogeneous hydrostatic stress (their internal pressure), we demonstrate, according to OFDR measurements on first composite vessels, the ability to detect some structural defects without the need to inflate them at pressures greater than their working pressure, thus reducing the risk of creation or evolution of new or existing internal defects.Besides the fact that these optical measurements provide information of a mechanical nature (i.e. the distributed strain profile all along the optical fibre), they are performed by the same sensor –the optical fibre– embedded into the composite structure throughout its lifetime, eliminating by design any measurement dispersion between different sensors.Thus, the optical fibre used in combination with OFDR turns to be a high-end and non-destructive in situ measurement technique for long-term SHM of high pressure vessels